WASHINGTON — U.S. retail sales fell sharply last month as gas prices plummeted and auto sales slowed. But excluding those volatile categories, Americans stepped up their spending a bit.

Retail sales dropped 0.8 percent in January, the Commerce Department said Thursday, following a 0.9 percent decline in December. The dollar value of gas station sales plunged 9.3 percent, the largest drop in six years. Auto dealer sales also fell for the second straight month after big gains in the fall.

Outside those categories, sales ticked up 0.2 percent after a flat reading in December.

The modest gain suggests Americans are still cautious about spending their windfall from lower gas prices, which economists expect will save the typical family $750 this year. Cheaper gas and strong hiring have sent consumer confidence to seven-year highs, a sign spending should pick up soon.

Economists were disappointed by the weak showing, but most expect that consumers will eventually spend much of the extra cash left over from lower prices at the pump.

"With lower gasoline prices leaving households with more to spend ... the labor market on fire and consumer confidence back at its pre-recession level, we had hoped to see a much stronger performance," Paul Ashworth, an economist at Capital Economics, said in a note to clients.

Ashworth now expects growth in the first three months of this year will be just 2.5 percent to 3 percent, below his earlier estimate of 3 percent or above.

Still, "the conditions are in place for a period of very strong consumption growth," he said. "We still expect to see that strength come through in the retail sales data soon."

The National Retail Federation, the nation's largest retail industry group, expects retail sales to be up 4.1 percent this year. That would be an uptick from the 3.5 percent increase last year and mark the biggest percentage gain since 2011 when retail sales rose 5.1 percent. The figures exclude business from autos, gas stations and restaurants and include online sales.

Jack Kleinhenz, the NRF's chief economist, said that heavy discounting will continue this year given the competitive environment and that, he believes, has made sales look less strong in January.

Adjusting for price changes, "demand has been stronger," he said.

Scott Hoyt, an economist at Moody's Analytics, said that consumers are frequently slow to spend "found money," such as tax rebates or savings from lower gas prices. It can take six to 12 months for the spending to fully materialize, he said.

One reason is that shoppers may not be sure that the low prices will last. A survey of consumers by the Conference Board, released Wednesday, found that 75 percent of respondents expect gas prices will rise by the summer. That's different than many analysts and government forecasts, which foresee gas below $3 a gallon for the rest of this year.

The Conference Board's survey also found that one-third of respondents will use their extra money from cheap gas to reduce debt or add to savings. Those moves could support more spending in the future. Only 20 percent said they will spend more on other items now, while 45 percent said cheaper gas will have no effect on their plans.

Some extra spending will likely go toward services, rather than buying more goods at retail stores. The Conference Board survey found that 40 percent of respondents were more likely to consider a road trip because of cheaper gas. That could lead to more spending on vacations.

Retail sales account for just over a quarter of overall consumer spending, with the rest going to services, including health care. When consumer spending is adjusted for inflation, which is falling on a month-to-month basis, spending data will likely increase.

Restaurants are a clear beneficiary: Sales at restaurants and bars climbed 11.3 percent in January from a year ago, the biggest 12-month gain in the past 33 years.

Yet some of the gain reflects a sharp drop a year ago, when restaurant sales fell due to heavy snowstorms in January 2014. That helped boost the one-year gain this month.

Excluding the volatile categories of autos, gas, building materials and restaurants, sales rose just 0.1 percent after falling 0.3 percent in December.

Consumers did spend more last month at electronics, building materials and general merchandise stores. But spending at clothing, grocery and furniture stores all fell.

Most recent economic trends point in the direction of more spending: Healthy hiring has meant that 3.2 million more Americans are earning paychecks than 12 months ago. Employers added more than 1 million jobs from November through January, the strongest three-month pace since 1997.

And average hourly pay rose 0.5 percent last month, the most in six years, the Labor Department said Friday. While economists cautioned against reading too much into one month's figure, it suggested employers may be starting to raise pay.

Overall, consumers ramped up their spending in the October-December quarter by the most in nine years.

Gas prices nationwide fell for 123 straight days to nearly a six-year low of $2.03 a gallon in January, according to AAA. That was the lowest in more than five years. Pump prices have since risen to $2.21 a gallon, but that is still $1.10 cheaper than a year earlier.

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AP Retail Writer Anne D'Innocenzio contributed to this report from New York.